It makes use of a number of Haskell libraries and systems such as Parsec, Cabal, Template Haskell, and monad transformers ("Your app then is just a set of state transformer functions (in the MACID Monad) that take an event and state as input and that evaluate to a new state, a response, and a (possibly null) set of sideeffects."[1]) among others.

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Happstack is the continuation of the HAppS project.[2] The project name was changed to highlight the transition to a more open development model and a change in project leadership. The transition was made with the blessing of Alex Jacobson, and the HAppS developers are still involved in the coding.